This invention relates to a substantially continuous method of making an optical filament transmission medium, and more particularly to a method of making optical waveguides of the type that are adapted to propagate only one or a few modes.
High capacity communication systems operating around 10.sup.15 Hz are needed to accommodate future increases in communication traffic. The systems are referred to as optical communication systems since 10.sup.15 Hz is within the frequency spectrum of light. Optical waveguides, which are the most promising medium for transmission at such frequencies, normally consist of an optical filament having a transparent core surrounded by transparent cladding material having a refractive index which is lower than that of the core. A very thorough and complete discussion concerning the operational theories of optical waveguides is contained in U.S. Pat. No. 3,157,726 issued to Hicks et al., and in the publication "Cylindrical Dielectric Waveguide Modes" by E. Snitzer, Journal of the Optical Society of America, Vol. 51, No. 5, pp. 491-498, May 1961. Another excellent source of information concerning optical waveguides is "Fiber Optics--Principles and Applications" by N. S. Kapany, Academic Press, 1967.
The propagation of light waves is governed by laws of physics similar to those that govern microwave propagation and therefore can be studied in terms of modes. Each of these modes has its own propagation and distribution characteristics. The conditions under which propagation of a particular mode will no longer be localized within and around the core of an optical filament can be expressed in terms of a cutoff value or parameter U. An exceptionally complex equation, and an explanation thereof, from which the value U for a particular mode can be determined may be found on page 55 of the aforementioned publication by N. S. Kapany. Kapany also expresses a fiber characteristic term R, now commonly referred to as V, in terms of the optical fiber variables by the equation ##EQU1## where .alpha. is the core radius of the waveguide, .lambda. is the wavelength of light to be transmitted, and n.sub.1 and n.sub.2 are the refractive indices of the core and cladding, respectively. As explained in Kapany, for particular mode to propogate in an optical filament having a particular filament characteristic V, V must be greater than or equal to the cutoff value U for said mode.
Typical multimode waveguides have core diameters between 50 micrometers and 100 micrometers and core-cladding refractive index differences of several percent. Thousands of modes propagate, each mode traveling at a slightly different group velocity. A short input pulse that is shared by many guided modes thus splits up into a sequence of pulses that arrive at the output end at different times. This pulse dispersion limits the information carrying capacity of multimode waveguides. The total number of modes that can be supported by a waveguide filament is given approximately by the equation EQU N=1/2V.sup.2 ( 2)
Equations (1) and (2) indicate that more modes can be guided if the core radius is large or if the refractive index difference is large. It is noted that equation (2) is not very accurate for small values of N, but it is useful for approximating the number of modes that will be propagated by a multimode optical waveguide.
It is possible to design an optical waveguide so that only one mode, the HE.sub.11 mode, is propagated, thereby eliminating the aforementioned mode delay distortion and opening the way to a gigabit transmission. It has been determined that for such single mode operation, V must be less than 2.405. If V is set equal to 2.405, and equation (1) is evaluated, it can be seen that a method of limiting light propagation of a desired wavelength to one mode is to coordinate the wavelength parameters a, n.sub.1, and n.sub.2. That is, if the difference between the two indices of refraction (n.sub.1 -n.sub.2) increases, the core radius must decrease, and if (n.sub.1 -n.sub.2) decreases, the core radius must increase. Producing waveguides having core and cladding indices of refraction within limits necessary to maintain single mode propagation is difficult even for waveguides with very small cores. The difficulty is markedly increased in producing waveguides with larger cores, since the difference in refractive indices must be correspondingly decreased. As an example, if the optical waveguide is to have a small core, that is, a core diameter of approximately 1 micron, the required difference in the two indices of refraction will be of the order of 10.sup.-2, and if the optical waveguide is to have a large core, that is, a core diameter of approximately 1 millimeter, the required difference in the two indices of refraction would be even smaller, that is, on the order of 10.sup.-4.
Even though single mode waveguides exhibit extremely low pulse dispersion, the use thereof for long distance transmission of light is not feasible unless they are capable of providing low loss light transmission. Absorption losses can be minimized by employing high purity glasses having an extremely low content of impurities. Although some intrinsic scattering due to inhomogeneities of the dielectric material is unavoidable, scattering losses also result from core-cladding interface irregularities caused by the trapping of numerous tiny air bubbles and foreign particles at the interface and by core diameter variations due to inadequate dimensional control. Furthermore, for single mode waveguides, a well defined core-cladding interface is required. The present invention relates to a substantially continuous method of forming an optical waveguide in which these above-mentioned scattering losses are minimized.